Diffractive grating structures are fine structures (cross-sectional profile e.g. 5 microns) of a surface that condition the passage of light based on physical optics including the diffraction effect. Many known ‘micro-prismatic’ structures are not true diffractive structures as they are relatively large (cross-sectional profile e.g. 50 microns), whereby altering the light passage is solely based on refraction effect therein, i.e. only the principles of geometric optics are applied.
Generally refraction of light at the interface between two media is described by the Snell's law:
                                                        sin              ⁢                                                          ⁢                              θ                i                                                    sin              ⁢                                                          ⁢                              θ                t                                              =                                                    n                2                                            n                1                                      =                                          v                1                                            v                2                                                    ,                            (        1        )            and reflection by θi=θr,wherein θi, θr and θl are the incidence, reflection, and refraction angles between the normal vector to the interface and incident, reflected and refracted beams, respectively. n's and v's refer to refractive indexes of first and second media and the speed of the light in the media, respectively. See FIG. 1a for visualization. Total internal reflection may occur when light arriving from material with higher refractive index with respect to the second material meets a medium boundary at an angle larger than the critical angle with respect to the normal to the surface.
A diffraction phenomenon is often inspected via a diffraction grating that refers to a transparent or reflective surface pattern of a large number of parallel lines, i.e. grating grooves or ‘slits’, used to disperse light into a spectrum. A transmission grating has grooves on a transparent material. A reflection grating has grooves on a reflective coating on a surface. Light rays that pass through or reflect from such surfaces are bent as a result of diffraction. The diffraction angle depends on the light wave length λ and so-called groove ‘spacing’ or groove ‘period’ d, sometimes also called as ‘pitch’, according to the following (in-plane) grating equation:mλ=d(sin α+sin βm),  (2)wherein angles α and β refer to incidence angle and diffraction angles, respectively. The angles are in this example measured from the grating normal. ‘m’ refers to the diffraction order that is an integer. For a selected wavelength λ the values of m complying with |mλ/d|<2 are obviously physically realizable in view of equation 2. Order m=0 corresponds to direct transmission or specular reflection. Each groove of a grating introduces a diffraction pattern, whereby many diffraction patterns by several grooves interfere. See FIG. 1b for schematic illustration of a reflective grating, wherein grating grooves are perpendicular to the page surface and a monochromatic light beam propagates in the surface plane. Most often diffraction the grooves of the diffraction gratings have rectangular cross-section, i.e. they are binary grooves, but also trapezoidal or triangular forms are possible.
Accurate and efficient light management is continuously growing in importance. Power consumption and light pollution are both actual issues in all optical applications. The main focus in optical solutions is currently in light coupling and directing structures.
Waveguides are elements that are especially adapted to carry waves within them. In optics waveguides are often called as lightguides and they typically include dielectric material with relatively high refractive index. When the waveguide is placed in an environment with lower refractive index the guide advantageously carries the waves according to the principles of total internal reflection. Lightguides include optical fiber, glass and e.g. different films.
Diffractive gratings such as binary grooves may be utilised in optical applications for light outcoupling and scattering. However, merely using the diffractive grating grooves doesn't automatically guarantee tolerable performance by any standard as to be reviewed hereinafter.
FIG. 2 discloses a scenario visualizing a cross-section of an elongated lightguide 202 whereto a light source such as a LED (Light Emitting Diode) 204 has been attached. Major part of the light is outcoupled from the lightguide top surface for non-preferred and non-suitable direction, when the incident angle thereof in relation to the media interface normal is smaller than the critical angle. As seen from the conoscope figure the light output emanating 206 from the lightguide 202 is neither particularly uniform nor directional. Such output is possible to attain by a number of microstructures provided to the lightguide bottom or top surface to alter the light path so that a part of light previously propagated in the guide with total reflection changes its direction and incident angle whereby it is transmitted outside the lightguide upon contacting the media interface.
FIG. 3 discloses a corresponding situation wherein a number of solitary, i.e. single, blazed grating grooves 302 have been positioned on a lightguide bottom surface 304 for outcoupling light. Because a blazed grating groove is particularly incidence angle selective and sensitive structure, the directive coupling efficiency of the grating groove is strongly related to the incidence angle of light, which is also the case with the conical angle, i.e. the angle between the incident light and the plane of FIG. 3 (XY) parallel to the grating normal. Thus directive outcoupling is achieved only with a very limited range of incidence angles in order to achieve narrow vertical outcoupling angles with still reasonable waveguide output coefficient, i.e. the preferred efficiency parameter determinable from the relationship between the incident and properly outcoupled light. With other incidence angles the light passes through the blazed grating structure and propagates further, either inside or outside the lightguide, for example. Major part of this light finally leaks out from the lightguide or is outcoupled at a non-preferred angle resulting non-directive output light and thus in many applications inferior lighting effect and low light coupling efficiency. One prior art solution with solitary blazed grating grooves, wherein the distance between consecutive grooves is at least several tens of microns, is disclosed in EP 1260853.
Notwithstanding the various prior art solutions, the output from the light emitting surfaces of lightguides or other lighting devices was not found fully adequate what comes to the directivity and uniformity of the outcoupled light. The outcoupled light was more or less Lambertian and non-uniform.
To further analyze the performance of diffractive grating grooves, several tests and simulations were conducted concerning the performance of solitary microprism-type grating grooves and differences between electromagnetic (˜physical optics) and ray tracing approaches when the grating grooves were located at a lightguide bottom as shown in FIG. 3. The following conclusions were drawn after analyzing interaction of light incident on a solitary grating groove or a micro-prism with 45° blaze angle thus having the shape of an equilateral triangle. In the test set-up light is incident onto a microprism-type groove at angle α ranging from 60° to 90°, where the angle α is the angle of incident light in the XY plane in relation to a vertical line. The angle θ is the conical angle, i.e. the angle of light incidence in the XZ plane. At θ=0° the light propagates perpendicularly to the groove, in the XY plane.
The aforesaid microprism-type grooves are generally suited to output light through the lightguide's top surface, if the angle α is in the range 87°<α<132° (corresponding to incidence angle 42°<γ<87° relative to the groove normal). With smaller angles the light will leave the lightguide through the grooves with only a small portion of light energy reaching the lightguide's top surface. The optimum angle α is 90°. When the angles α range from 80° to 100°, the output angle from waveguide is substantially no greater than 15°. It was further found that for a relatively narrow range of angles α, e.g. from 87° to 100°, while recalling that for angles less than 87° some light passes through the diffractive groove, the waveguide output coefficient is clearly over 0.90, when the coefficient is defined as a product of the groove's reflectance by the transmittance of the waveguide's top surface. Deviation of θ from zero leads to an increase in the absolute value of the output angle φ. For θ greater than 10° it is practically impossible to get the light outcoupled from the lightguide top at angles smaller than 15°. A solitary groove is thus not particularly suitable for outcoupling light with wide incidence angle perpendicularly to the exit surface.
By comparing the electromagnetic modeling incorporating e.g. the polarization phenomenon with the ray tracing modeling (geometric optics) it was found that the two approaches may provide rather equal results concerning the propagation direction of light and light output coefficient when the angle θ is not especially large, e.g. <30°. However, electromagnetic modeling provides a higher light output coefficient due to diffractive phenomenon, especially in connection with small and large incidence angles.